Irradiation induced apoptosis is realized through mitochondria-dependent mechanisms that release proapoptotic factors from mitochondria into the cytosol where activation of caspases triggers irreversible hydrolytic stages of the apoptotic program. Thus, mitochondrial segment of apoptosis may be a particularly attractive target for preventive/therapeutic interventions provided the mechanisms of mitochondrial damage are elucidated. We discovered that, during chemically induced apoptosis, interactions of cytochrome c (cyt c) with a mitochondria-specific phospholipid, cardiolipin (CL), yield CL/cyt c complex that functions as a peroxidase activated by reactive oxygen species (ROS) generated by (damaged) electron carriers. This causes the formation of oxidized CL (CLox) which is required for the release of pro-apoptotic factors from mitochondria. We established that irradiation induced apoptosis triggers selective oxidation of CL both in cell cultures and in vivo. Thus, our central hypothesis is that irradiation induced damage of mitochondrial electron transport results in excessive production of ROS (H2O2), activation of CL/cyt c complex, and accumulation of CLox resulting in the release of pro-apoptotic factors from mitochondria. As a corollary to the hypothesis, we predict that irradiation induced apoptosis can be prevented by: a) acceptors of electrons from mitochondrial electron transporting complexes which are capable of inhibiting ROS production in mitochondria; b) quenchers of peroxidase activity of CL/cyt c complexes, such as NO>-donors; c) transgene antioxidant enzyme interventions capable of inhibiting CL oxidation; d) non-enzymatic free radical scavengers/lipid antioxidants; and e) treatments reducing CL susceptibility to oxidation by (dietary) manipulating unsaturation of its fatty acid residues. In the proposed project, we will validate our hypothesis and develop, synthesize, screen, and select optimized combinations of chemical/dietary interventions with maximal preventive/mitigating power at different times before and after irradiation. Combinations of small molecules including electron acceptors, inhibitors of peroxidase activity and lipid antioxidants may be effective at very early stages after irradiation (minutes to hours). Transgene antioxidant manipulations may take longer time to reach the maximal effects (few hours to days). Relatively non-toxic nutritional manipulations may require several days for changing CL fatty acid composition in critical organs, but they are particularly promising for long-term mitigation and/or prevention of irradiation induced apoptosis.